The electrons in the weak p-bonds can
be used to form strong s bonds to other monomer units.

What kinds of structural changes
accompany bond-breaking and bond-forming in olefin
polymerization?

Answer:

The olefin monomers are flat
(two-dimensional) molecules with sp2-hybridized
carbon atoms. The polymers are three-dimensional
molecules in which the carbon atoms are sp3
hybridized.

Examine samples of LDPE (sandwich
bag, squeeze bottle) and HDPE (milk jug, grocery bag).
What are some of the differences in the physical
properties of these substances?

Answer:

LDPE  more transparent,
flexible, waxy

HDPE  more opaque,
rigid, non-waxy

How does the molecular-level
structure of these polymers influence their physical
properties?

Answer:

The structure (e.g., extent
of branching) determines how the individual polymer
molecules can orient (or "pack") in the
solid state. This, in turn, influences physical
properties such as density, crystallinity, melting
point, and strength.

How can chemists control which type
of polyethylene (LDPE vs. HDPE) is generated?

Answer:

Through the choice of
appropriate catalysts and reaction conditions.

Besides the extent of branching, can
you think of any other structural parameters that might
lead to differences in physical properties?

Answer:

The average value of n (the
number of monomer units in the polymer) and the range
in individual values of n.

Does ethylene polymerize under mild
conditions in the absence of a catalyst?

Answer:

No, in the absence of a
catalyst, ethylene molecules would need to collide at
very high energy in order to react with each other.

What is the role of a catalyst?

Answer:

A catalyst reduces the energy
of activation for a reaction by providing an
alternative pathway. In this way, it speeds up the
reaction and allows it to proceed under milder
conditions.

Why are metals often good catalysts?

Answer:

They provide a site where
organic molecules can come together and react.

What is the difference between a
heterogeneous and a homogeneous catalyst? What are some
of the advantages of homogeneous catalysts?

Answer:

Heterogeneous catalysts are
insoluble in the reaction medium, while homogeneous
catalysts are soluble. Since homogeneous catalysts
are generally molecular species, they are more
amenable to study using the spectroscopic tools of
chemistry. In addition, they can be chemically
modified or "tailored" to produce polymers
with a particular kind of structure.

How would you describe the
orientation of the ligands around the Zr center in the
homogeneous zirconium catalyst?

Answer:

The ligands  the two
cp's, the alkyl group, and the olefin (or open site)
 are oriented in a tetrahedral fashion around
Zr.

What is the nature of the bonding
interaction between a metal and an olefin?

Answer:

The olefin uses the electrons
in its p-bond to
interact with the metal.

Polymer chain growth can be
terminated by b-hydride
elimination or by reaction with H2. What is
one advantage of the H2 reaction?

Answer:

It allows the chemist to stop
chain growth at a desired stage, rather than relying
on the "natural" process of b-hydride
elimination. Hence, it gives the chemist some control
over the value of n.

Write out the first few steps of
polymer growth, using cmpd. 9 as catalyst.

In Session 2, we learned that
chemists can control the properties of polyethylene by
choice of appropriate catalysts and reaction conditions.
Based on what you have learned in Session 4, what are
some other ways in which chemists can manipulate the
properties of polymers?

Answer:

By using different
monomers. For example, the incorporation of a
phenyl (C6H5) unit into
the monomer leads to polystyrene, while the
incorporation of a chloro (Cl) group leads to
PVC, a polymer with very different
properties.

By using different
fabrication techniques. For example,
polystyrene can be glassy or foamy depending
on how it is fabricated.

"Teflon" is the polymer
that results from the polymerization of
tetrafluoroethylene. Write a chemical formula for this
reaction. What are some of the properties of Teflon?

Answer:

n(CF2
= CF2) ® (CF2CF2)n. Teflon is highly resistant to
chemical attack and has a very low coefficient of
friction (it is slippery). In addition, it can be
used over a very wide temperature range (-73
oC ® 260 oC).

"Co-polymers" consist of
two different monomers ("A" and "B")
joined in an alternating fashion (ABABAB...). Block
co-polymers also consist of two different monomers, but
in this case blocks of polymer containing only A
units are joined to blocks of polymer containing
only B units (AAAAABBBBB...). How might block co-polymers
be synthesized?

Answer:

After the polymerization has
been allowed to proceed with monomer A, the olefin
feedstock is changed to B and the polymerization
continues.

Head-to-tail polymerization of
propylene is observed with the [cp2Zr(R)]+
catalyst. Explain this result on the basis of
molecular-level interactions.

Answer:

Each incoming propylene
molecular orients with its methyl group in toward R,
rather than out toward cp, in order to avoid
unfavorable contacts with the bulky cp's. When the R
group migrates to propylene, it migrates to the
closer olefinic carbon, which is always the one
bearing the methyl group (the "b
carbon").

Atactic polypropylene is always
produced with the [cp2Zr(R)]+
catalyst. Explain this on the basis of molecular-level
interactions.

Answer:

There is no preference for
the methyl group on propylene to be oriented up or
down, because in each case it has exactly the same
interaction with a cp group. Since there is no
up/down preference, a random (atactic) orientation of
methyls along the chain results.

What does it mean for a molecule or
a ligand to be "chiral"? What properties does
chirality impart to a molecule?

Answer:

Molecules that are not
superimposible on their mirror images are chiral.
Mirror image isomers are called enantiomers.
Enantiomers have identical physical properties except
that they rotate plane polarized light in opposite
directions.

Explain what is meant by "C2
symmetry" and "mirror plane symmetry". Can
a molecule with "mirror plane symmetry" ever be
chiral?

Answer:

"C2 symmetry" means
that 180o rotation about an axis through
the molecule results in a geometry equivalent to the
starting geometry. "Mirror plane symmetry"
means that one half of the molecule can be perfectly
reflected into the other half through a symmetry
plane. Molecules with mirror plane symmetry cannot be
chiral.

Consider a homogeneous zirconium
catalyst in which two cyclopentadienyl ligands are
connected by a CH2CH2 bridge.
Which symmetry elements does the (bridged-cp2)Zr moiety
possess? What is the expected tacticity of the
polypropylene produced using this type of catalyst? Why?

Answer:

The (bridged-cp2)Zr
moiety possesses both C2 and mirror plane symmetry.
Like the parent unbridged cp2Zr catalyst
discussed in Session 5, it would be expected to
produce atactic polypropylene, since the methyl group
on the propylene would have no up/down preference.